This invention relates to the manufacture and structure of a radio frequency (RF) antenna, to a compact antenna element for use in a compact array antenna.
As is known in the art, there is a trend to provide increasingly compact RF antennas for use in radar systems used in airborne or land based seekers, including, but not limited to, direction finding (DF) systems and airborne vehicles (e.g. airplanes and unmanned vehicles).
As is also known, it is relatively difficult to provide compact antennas having high gain and large bandwidth and which are also relatively easy to manufacture at a relatively low cost. Current state of the art struggles to accomplish all of the above in one design. One prior art example of a solution to this problem is found in U.S. Pat. No. 6,052,889, to Yu, et al., (Yu '889). In that apparatus, the inventors attempt to address the above-noted need for an inexpensive compact, high grain, large bandwidth antenna by injection molding a group of broadband RF radiating elements from a polymeric material, metalizing each broadband radiating element, and installing a transmission line within each radiating element. While this design provides excellent antenna performance, it requires a complicated manufacturing process.
As is also known, conventional electric ground planes limit the minimum height of an antenna to one quarter of a wavelength since the current images projected onto the electric conductor that forms the ground plane are 180 degrees out of phase. Antenna currents that are less than a quarter of a wavelength from the electric conductor start to “short out” with their respective images, resulting in poor radiation efficiency.
To overcome this problem, electromagnetic bandgap (EBG) structures and materials (also referred to as photonic bandgap material or “metamaterial”) have been used. EBG structures have been utilized in commercial devices such as cell phones to aid in antenna size reduction. The use of an EBG ground plane is different than a traditional electric ground plane (e.g. a perfect electrically conducting (PEG) ground plane) since an EBG ground plane essentially acts like a magnetic conductor. Since image currents induced onto a magnetic conductor are in-phase with antenna currents, the antenna's height is no longer restricted and antenna features can now reside just above the ground plane while still providing good radiation efficiency. Thus, the use of an EBG material in a ground plane allows antenna elements to be placed very near to the ground plane without shorting the element.
The use of similar, alternate ground plane materials (specifically, a high-impedance metallic surface) is discussed in U.S. Pat. No. 6,545,647 to Sievenpiper, et al., incorporated herein by reference in its entirety.
U.S. Pat. No. 6,952,190 to Lynch, et al. (incorporated herein by reference in its entirety) discusses so-called “low profile” slot antenna using a backside fed high-Z material in the vein of Sievenpiper.
The use of EBG metamaterials as a ground plane for an Archimedean spiral antenna is described in Jodie M. Bell and Magdy F. Iskander, “A Low-Profile Archimedean Spiral Antenna Using an EBG Ground Plane,” IEEE Antennas And Wireless Propagation Letters, vol. 3, 2004, incorporated herein by reference in its entirety. Similarly, U.S. Pat. No. 6,175,337 to Jasper, et al., (incorporated herein by reference in its entirety) describes a photonic bandgap as a “high-impedance electromagnetic structure” used in a slotted waveguide antenna.